(1) Field of the Invention
The present invention relates to the field of nuclear medicine. Particularly, the present invention relates to the field of transmission scanning to provide nonuniform attenuation correction within a gamma camera system.
(2) Background of the Invention
Non-uniform photon attenuation is an important factor that affects the quantitative accuracy of images collected using Single Photon Emission Computerized Tomography (SPECT) camera systems and can decrease the sensitivity of these systems for lesion detection. Nonuniform photon attenuation creates image degradation by interfering with and partially absorbing the radiation emitted from an organ containing a radio-pharmaceutical. Photon attenuation within SPECT systems tends to degrade images by also introducing image artifacts and other distortions that can result in false positive detection or the undetection of lesions. The effects of photon attenuation are especially complex in cardiac studies as a result of nonuniform attenuation attributed to the thorax.
Typically, no attenuation correction is employed in myocardial SPECT, making it difficult for the physician to accurately diagnose lesions in cases where the patient is large-breasted, or where the diaphragm lies over the heart. Some investigator's have used attenuation maps which assume uniform density inside the body contour; however, the uniform maps have tended to introduce artifacts into the reconstruction images. To correct for non-uniform attenuation due to the breasts, lungs and diaphragm, some investigators and manufacturers employ radionuclide transmission sources mounted opposite the gamma camera detector to directly measure the attenuation factors for each patient. These factors may then be used in conjunction with an iterative reconstruction process to correct for non-uniform attenuation.
Transmission computed tomography (TCT) can be used as a method for generating a nonuniform attenuation correction distribution. The transmission image data is gathered using a known source (e.g., line, sheet, or flood) of radiation. If performed separately from the SPECT emission study, the collection of the transmission data requires additional data acquisition time and the collection of the transmission and emission data is susceptible to misregistration effects due to patient (e.g., "object") movement between the data gathering sessions.
In transmission scanning, the source of radiation is directed toward the associated scintillation detector through the object of interest or patient. If the radiation field is significantly larger than the patient, the radiation source is allowed to directly radiate the detector, causing a high count rate in the scintillation detector. Those parts of the detector that become directly radiated are called unobstructed portions of the detector. It is not advantageous to allow large unobstructed detector areas because the resultant increase in count rate can lead to image degradation and in some cases the event detection electronics and processes can become overloaded (e.g., due to pulse pile-up) and temporarily terminate operation. These high count rates tend to reduce the imaging performance of the imaging system by loading down the signal detection and processing circuitry of the gamma camera.
In the prior art, fixed "bow tie" filters were used in conjunction with line sources for transmission operations. The bow tie filter is used to reduce the count rate over detector areas that are unobstructed by the object being scanned. However, the use of a fixed bow tie filter is not advantageous because variably sized patients create differently sized unobstructed areas on the detector. A fixed bow tie filter is not advantageous because it will not account for patient variance and further requires effort to exchange in order to install new filters.
Also, during ECT scanning, different portions of the object become within the detector's field of view at different rotation angles, therefore different parts of the detector become unobstructed from the line source at different ECT angles. It would be advantageous, then, to provide a system to account for variable sized patients and also to account for different unobstructed portions of a detector for given ECT rotation angles during transmission sessions. The present invention provides such advantageous functionality.
Accordingly, it is an object of the present invention to more effectively improve image quality within a nuclear medicine camera system. To this end, it is an object of the present invention to provide a filter configuration to account for variably sized patients in order to effectively reduce the count rate in unobstructed portions of a radiation detector during transmission imaging. It is another object of the present invention to provide a filter configuration to account for the variable unobstructed portions of a scintillation detector that become present as the detector undergoes ECT angles of rotation during transmission scanning for ECT sessions. It is an object of the present invention to provide a variable bow tie filter configuration within a filter assembly wherein different filters can automatically be installed for use in time varying circumstances. These and other objects of the invention not specifically recited above will become clear within discussions of the present invention herein.